[syft.js] Handle large messages in data channel

[vvmnnnkv]

Data channel is limited to a certain size (depending on the browser) with the safe max value of around 16kb.
Update webrtc.js's sendMessage to handle large binary messages, such as protobuf blobs.

This involves:

• Split large messages into parts
• Basic protocol for split data, like header and number of parts (+ we don't need to split small messages)
• Control of bufferedAmount
• Concat on the other side and error checking

Example:
https://webrtc.github.io/samples/src/content/datachannel/datatransfer/

[vvmnnnkv]

Resolved by #82